The Accuracy of Mechanical Axis Measurements with an Image-based Robotic Total Knee Arthroplasty System

Introduction Studies that examine the image-based Rosa robotic knee arthroplasty system in restoring the desired mechanical axis (MA) are rare. The purpose of this study was to measure the accuracy of an image-based robotic knee system in restoring the desired MA. Methods Inclusion criteria was patients undergoing an imaged-based total knee arthroplasty with preoperative and postoperative long-leg standing radiographs. Six measurements were analyzed: 1) preoperative radiographically-determined MA (PreopRadMA), 2) preoperative industry/software-determined MA (PreopSoftMA), 3) robotically-determined native MA (PreopRobMA), 4) intraoperatively-planned MA (PlanMA), 5) intraoperative MA after-implant-cementation (IntraopMA), and 6) 2-week-postoperative radiographic MA (PostopRadMA). An outlier was defined when there was greater than 3⁰ between MA measurements. Results There was excellent agreement among preoperative measurements, poor-to-moderate agreements between planned MA and IntraopMA and PostopRadMA, and good-to-excellent agreement comparing IntraopMA with PostopRadMA. The IntraopMA and PostopRadMA deviated from the PlanMA by 1.4±1.1⁰ (4% outliers) and 1.6±1.3⁰ (14% outliers), respectively. The PostopRadMA differed from the IntraopMA by 1.5±1.2⁰ (8% outliers). There were greater differences found in the mean variances and outlier percentages between the 1) PreopRadMA and PreopRobMA (2.8±2.0⁰; p<0.0001; 40% outliers;) and 2) PreopSoftMA and PreopRobMA (3.3±2.2; p<0.0001; 50% outliers) compared to the mean variances and outliers between the PostopRadMA and IntraopMA (1.5±1.2⁰; 8% outliers). There was no difference between the accuracy of the PostopRadMA (1.4±1.3⁰) and the PlanMA (1.6±1.4⁰; p=0.37) or the IntraopMA (1.5±1.2⁰; p=0.63) in varus compared to valgus knees. Conclusions The mean differences in planned MA with IntraopMA and PostopRadMA were less than 2⁰. Intraoperative measurements may be more clinically relevant predictors of final alignment than planned MAs. There was decreased variability between the postoperatively-determined MAs compared to preoperatively. There was no difference in the accuracy of obtaining the planned MA postoperatively based on the type of preoperative malalignment (varus/valgus).